The present invention relates to a method of producing unbaked agglomerates such as iron-making cold pellets or briquettes by using such raw material as iron ore fines, minus sieve or undersize baked pellet powder or dust powder produced in an iron work.
Generally, the production of unbaked agglomerates, e.g. cold pellets or briquettes for iron making is suited for the disposal of single-grade powder material such as iron ore fines or undersize powder produced during the baking of pellets or the disposal of a large amount of dust discharged from an iron works.
Such colds pellets, briquettes or the like must posses the desired handling strength as well as the desired strength during the gas reduction. Particularly, when used in a gas reducing furnace, it is essential that the material possesses a sufficient strength and has no danger of swelling during the reduction. In the process of reducing and melting the material at elevated temperatures as in the case of a blast furnace or an electric furnace for making pig iron, there is no difficult problem from the standpoint of strength since the material passes relatively quickly through a temperature region where its strength is decreased during the reduction and enters rapidly into a high temperature zone where the sintering takes place. Also, in the case of a shaft reducing furnace or the like, the material is reduced with a low-temperature high-reducing gas as compared with the blast furnace and moreover the product which is decreased in strength by the reduction is charged as such into the following-stage electric furnace. As a result, the unbaked agglomerates must have a sufficient strength to retain their shape during the time that they are handled.
The conventional unbaked agglomerates made by using only iron ore fines or undersize powder of baked pellets as a raw material are usually low in strength during and after the reduction. As a result, they are swollen and powdered in the gas reducing furnace and it is not possible to produce agglomerates having a sufficient strength for charging into the eletric furnace.
In other words, the cold bonded pellet process is suited for the agglomeration of iron ore fines or dust particles formed and recovered during the smelting and it is attracting notice as an energy saving process involving no environmental pollution since it requires no baking in contrast to the sintering process.
However, while, as regards the reduction of cold bonded pellets produced from raw materials of iron oxide types by the cold bonding process, no serious difficulty will be caused if the cold pellets reach a high temperature reducing zone in a relatively short period of time and the reduction proceeds as in the case of a large blast furnace, in the case of a gas reducing DR furnace (Midrex, Hyl, etc.) which performs the reduction at a relatively low temperature, there are many instances where the pellets are subjected, as incidental to the low temperature reduction, to swelling, reduction powdering, etc., and the resulting reduced iron (DRI) is not suited for use in the following smelting operation. This tendency becomes more marked as the reducing temperature is reduced and as the raw material used is higher in iron ore grade and of the hematite type.
As mentioned above, if the cold pellets produced from iron ore fines or undersize powder of baked pellets are reduced, reducing powdering and swelling of the pellets occur in a relatively low temperature range between 700 .RTM. and 1000.degree. C. and this tendency frequently occurs in the case of raw materials such as high quality hematites. This tendency is also marked in the case of reducing furnaces, particularly direct reducing furnaces where the reduction is performed by a highly reducing gas of a relatively low temperature.